An emitter of this type serves as an electron source and is arranged in a cathode of an X-ray tube. The electrons generated by the flat emitter by resistance heating, e.g. by current feed (application of filament current), are accelerated in the direction of an anode (target). When the electrons collide with the anode they are braked, wherein X-ray radiation arises, which can be used for example for diagnostic imaging, for therapeutic irradiation, for analytical material examination or for a safety review.
During operation of the X-ray tube a filament current (resistance heating) is applied to the flat emitter, which is preferably made of tungsten, tantalum or rhenium, and it is thereby heated to temperatures of up to 2,600° C., as a result of which electrons can, because of their thermal motion, overcome the characteristic work function of the emitter material and then be available as free electrons. After their thermal emission the electrons are accelerated onto an anode by an electrical potential of approx. 120 kV. When the electrons strike the anode, X-ray radiation is generated in the surface of the anode.
The flat emitter can in each case be mounted rigidly in the cathode head on the two connection legs via which the filament current can be supplied.
The temperatures occurring during operation lead in the case of the flat emitter to relatively strong linear expansions which because of stresses result in elastic and/or plastic deformations, wherein mechanical stresses of between 100 MPa and 200 MPa can occur on the emitter because of the thermal expansion. Plastic deformations can have negative impacts on the geometry of the emitted electron beam, meaning that the geometry of the focal point generated on the anode and consequently the image quality may be correspondingly degraded. In addition, the constant activation and deactivation of the filament current during operation of the X-ray tube results in an alternating fatigue loading of the thermionic emitter, which dramatically reduces the service life of the emitter.
Flat emitters with a rectangular emitter surface are described for example in DE 27 27 907 C2 and DE 10 2008 046 721 B4. A flat emitter with a circular emitter surface is known from DE 199 14 739 C1. In the case of the flat emitters cited the emitter surfaces are in each case electrically contacted in a cathode head via two band-type connection legs. The emitter surface and the two band-type connection legs are embodied integrally in the case of the aforementioned flat emitters and are brought into a 90° position via a bend and fixed rigidly in the cathode head. Because of a certain inherent elasticity of the connection legs, there is a limited elasticity of the suspension of the flat emitter. However, there is a risk that when installing the flat emitter the band-type connection legs are torsioned. In the case of a flat emitter with a rectangular emitter surface the band-type connection legs then lie in line with the expansion direction of the emitter. As a result the rigidity of the band-type connection legs deteriorates.
Furthermore, a flat emitter is disclosed in U.S. Pat. No. 7,693,265 B2 which has rigid rod-shaped connection legs (support rods) welded to its rear side.
U.S. Pat. No. 6,801,599 B1 further describes an emitter with welded-on contact rods, in which a certain amount of flexibility can be achieved when fixing the emitter in the cathode head thanks to long sleeves.
A further flat emitter is known from US 2014/0239799 A1, which comprises a rectangular emitter surface which emits electrons when a filament voltage is applied. On one side of the emitter surface the flat emitter has a first end region and on its other side a second end region. A first connection leg is arranged in the first end region and a second connection leg in the second end region. Both connection legs of the flat emitter have a cylindrical geometry, and are thus embodied as rod-shaped and are fixed to the rear side of the flat emitter by means of a material-fit connection in each case (welded or soldered connection). The connection legs thus form support rods for the emitter surface of the flat emitter. Because of the cylindrical geometry the connection legs are easy to manufacture and during installation are invariable in respect of a torsion about the cylinder axis. The disadvantage of the cylindrical geometry is a high level of rigidity (spring rigidity and torsion rigidity) of the connection in the focus head. If the rigidity is too high an excessive restoring force of the connections arises because of the longitudinal expansion of the flat emitter and may result in damage to the flat emitter.
To prevent thermally conditioned longitudinal expansions of the flat emitter from resulting in an elastic and/or plastic deformation of the flat emitter and thus of the emitter surface, it is known from DE 10 2010 039 765 A1 for a flat emitter to be positioned in a first end region via a fixed bearing and to be restricted to a thermal main expansion plane in a second end region via a sliding bearing. In this solution, which is relatively complex in terms of design, thermal longitudinal expansions of the flat emitter thus do not exert any negative effects on the geometry of the emitted electron beam.